"Fuse Resistors" on Power Tube Cathodes?

[Alexo]

What's the expert's take on these?

The conundrum is to pick a resistor wattage small enough to blow in the event of a short in the tube, but large enough not to blow when pushed night after night in the bar.

What's throwing me off is that I can't think of an amp where the PT has enough surplus current to actually take advantage of this.

For instance, I have a 100 watt amp on the bench running a pair of 6550's. According to my load line, the power tubes should swing all the way up to 400-450ma. The PT is rated at 465 ma.

So if we put a 3watt 10 ohm R in there, it should dissipate close to 2 watts when the amp is cranked (a bit closer than I'm comfortable with) and blow out if things exceed 550ma. ...but that PT is never going to sink 550 ma! The filter caps might push it over the edge, but just for an instant, probably not long enough to blow the R, I'm thinking.

...makes me think that this is why we mostly see this in the hifi world, where they run stereo power amps - the PT is spec'd for double the current and *could* divert enough current into a shorted tube to pop the Resistor open.

Just seems kinda silly to have that resistor in there, so close to its breaking point for a benefit that may or may not happen.

What about just using a couple diodes in parallel, one oriented cathode to ground and the other anode to ground, so if things ever exceed 1A, kapow!

...?

[JamesHealey]

Can you depend on a component to blow at it's rating?

[Phil_S]

Use a fuse for a fuse, not a resistor. I'm all for innovation and expansive thinking, but not on this point.

[Alexo]

Use a fuse for a fuse, not a resistor. I'm all for innovation and expansive thinking, but not on this point.

Yeah, honestly, it all feels a little cockamamie. I'm thinking an HT fuse on the PT CT and flyback diodes might be sufficient.

[Structo]

I don't know about a 6550 amp but I have heard of guys using 1/2 watt 1 ohm cathode resistors and not have them blow when playing loud.

Maybe the quenching diodes ala Ken Fischer and Ampeg are worth a look see?

[Dingleberry]

What is a quenching diode?

In some amp schematics I have seen a diode parallel with those 1 Ohm resistors.
Does quenching diode mean that?

I've used 1 Ohm 1/2W 1% presicion resistors and never experienced any of them failing.

-T

[FYL]

What is a quenching diode?

Flyback diode. This has been discussed many times here.

I've used 1 Ohm 1/2W 1% presicion resistors and never experienced any of them failing.

P = I^2 * R, you need at least 700 mA of current thru a 1R to reach 1/2 W diss.

[Bob-I]

Resistors power rating is calculated as the minimum it can sustain based on test criteria. It may vary according to the test. For example, the 1 watt Vishay-Dale resistors are rated 1/2 watt for the military. Do not use a resistor as a fuse.

[David Root]

+1 on not using resistors as fuses. Especially not the 1 ohm cathode resistors that allow handy measurement of cathode current.

I still recall the true story of a tech who put 1/4W 1 ohm resistors in a client's amp. One of them blew mid-performance--in Europe.

I use Mills wirewound 5W 1 ohm resistors in that spot. Expensive, but they are small enough to fit reasonably on a 3/4" span of the meter test jacks, and accurate enough too. 5W sandboxes don't cut it, they have to be mounted too far out and look like lumpy hemorrhoids.